Spray-coating method

ABSTRACT

A method for spray-coating substrate surfaces, in which
         (a) a thermoplastically processable material is molten and thus liquefied in an extruder in a first step;   (b) the molten material is pressurized by means of a carrier gas or vapor;       

     (c) a mixture formed from said molten material and said carrier gas or vapor is pressed through one or more nozzles, optionally supplying a spraying gas having a temperature at least as high as the melt temperature to the spraying jet in the region of the discharge opening of the nozzle; and
         (d) the obtained spray jet with the molten material is directed onto the substrate surface, wherein the material impinges in drops in a flowable state onto the substrate surface, forms a continuous coating on the substrate surface, and subsequently solidifies.

FIELD OF THE INVENTION

The invention concerns a method for spray-coating substrate surfaces which enables different thermoplastically processable materials to be applied to very different types of surfaces by means of spray technology.

BACKGROUND

Very different types of methods are known for the preparation of thin-walled sheet-like components or, for example, insulations on textiles. Depending on the application, these methods differ in shape quality, material, thickness distribution over the substrate (component), and the technology itself

Known methods include these of deep-drawing, vacuum deep drawing and pressing for the processing of sheets into formed parts or individual layers.

In the production of sheet-like components by means of such methods, the local mass distribution is determined by the deformation of the sheet and cannot be set in a defined way. For this reasons, for example, thus produced insulations are heavier than necessary from a functional acoustic point of view. This prevents the sought lightweight construction especially in vehicles.

For material sealing, sheets are applied to very different types of substrates by adhesive or hot-melt bonding.

For partial sound insulation, from an acoustic point of view so-called heavy layer sheets are placed on carpet or bulkhead zones, where they are attached by adhesive or hot-melt bonding.

Other methods are known for producing thin sheet-like components for motor vehicles, such as heavy layer components for acoustic insulation components, including bulkheads, produced by injection molding of thermoplastic and thermosetting materials.

Injection molding (thermoplastic injection molding or reaction injection molding (RIM)) allows components with different defined masses per unit area to be produced. Because of the high investment in the plants and molds, high quantities must be produced by these methods.

For the RIM method, there is the additional problem that the material to be employed, for example, polyurethane, is expensive and cannot be recycled. Especially for insulation components in vehicles, the mass distribution for all superstructures is determined by the mold geometry. This also prevents the purposeful acoustic optimization of individual vehicle types (engine versions) within a series, which would be desirable in acoustic terms.

Further, methods for producing such sheet-like components by the sintering of powder are known. The powder is applied to a warm molding tool, where it forms a plastic layer by sintering, followed by cooling and withdrawing the component. This technology is intensive in time, tools and energy. Thus, this method remains limited to the production of high-quality components, such as slush-molded skins.

Methods for the coating as well as production of sheet-like components by spraying are known. The spraying of thermoplastic materials is effected either from a melt by means of one or more nozzles, or via a “cold” plastic powder by means of heating above a flame during the flight phase and a carrier gas, or directly by hot surroundings during the flight phase. However, these methods have very low throughputs and are only conditionally suitable for the coating of substrates, i.e., only for temperature-resistant ones. In the coating of textiles, the high temperatures would destroy the textile material before application is completed.

The spray method also finds application in powder regeneration. DE 10 2005 050 890 A1 describes a method and device for producing a nanocomposite in which the separation of extractant and dispersant, if any, from a polymer melt is effected by a high pressure spraying method. In this method, the polymer droplets solidify abruptly as a consequence of the rapid cooling taking place in the relaxation of the high pressure level to environmental pressure. This generates the nanocomposites.

As a heavy layer for spraying, cross-linking PUR-based systems are employed. Details are found in DE 101 61 600 A1 and DE 10 2005 058 292 A1. These materials are very expensive and cannot be recycled.

Further, methods and devices are known in which a plastic bead is laid onto a, mostly pretreated, substrate surface. EP 0 524 092 B2 describes a method and device for producing an article with a profiled cord. The profiled cord is produced by an extrusion die connected with the extruder through a heated flexible pressure tube, and laid down.

DE 30 47 727 C2 describes a method for producing thin protective films by spraying liquefied thermoplastic material onto a substrate. The method described herein is supposed to enable the use of conventional hot spraying devices, in which injection material is molten and thus liquefied, which can then be applied immediately to the surface to be protected. It is further described that the tendency of these materials to form beads upon impinging on the surface has until now prevented the formation of a sufficiently homogeneous film, but according to this description, this problem is supposed to be overcome in the simplest way by a simultaneous or subsequent sintering process. Accordingly, for the sintering of the thermoplastic material, it has been found advantageous if the sintering is effected by heating the sprayed-on thermoplastic material in order to arrive at a uniform, smooth and pore-free application. The heat required for sintering can be supplied to the thermoplastic material externally by radiant heat, warm air supply or the like. It may be reasonable to spray the thermoplastic material onto a previously heated surface, so that the sintering process occurs simultaneously with the spraying, and thus a time-optimized course of the process is obtained. However, the sintering process effected after the material's impinging on the surface is very complicated and again puts the substrate material under a thermal load.

DT 16 46 051 B2 describes a method for applying polymeric coatings to solid surfaces by spraying on a molten thermoplastic polymer. Thereafter, melting of the polymer and supplying the melt in a pressurized gas stream by relief embossing are performed, and the sprayed gas/polymer jet is subjected to heat jet treatment on its way to the surface to be coated. It is described that the thermoplastic polymer of any grain size is converted to the molten state in extruders or piston cylinder means, and sprayed onto the surface to be coated, which is previously heated, by means of pressurized gas using a compressed air injection nozzle in the form of a gas/polymer jet heated by a heat jet stream.

DE 32 25 844 A1 describes a method and device for the application of layers of thermoplastic materials or hot-melt adhesives. A method for the application of layers of thermoplastic materials or hot-melt adhesives is described in which the plastic or hot-melt adhesive employed is molten, then atomized and sprayed. Preferably, the temperature of the molten plastic or hot-melt adhesive is kept constant up to the moment of spraying. The device for performing the method includes a heatable melting means for a plastic or hot-melt adhesive, a heatable spraying means having a nozzle, a heatable device for supplying the molten material into the spraying device, a temperature measuring and temperature closed-loop control system, as well as closed-loop control systems for the supply and discharge of a molten material and the heated spraying gas, if any. By this method, two- or three-dimensional objects of any kind and shape can be provided with an arbitrarily thick, uniform or patterned, layer from the outside and/or inside.

DE 42 31 074 A1 describes the use of plastic powders as a filler in sprayable coating compositions, paints and sealing compositions. It describes the use of powders of a density range of from 0.1 to 2.0 g/cm³ having an average grain size of at most 0.2 mm, obtained by mechanical comminution of solid plastic material that may contain mineral fillers, as fillers, optionally in addition to further fillers, in sprayable paints, coating and sealing compositions based on one- or two-component polyurethane binders.

DE 101 61 600 A1 describes a method for spraying on plastic layers. It describes a method and device for applying a filler-containing plastic layer to a shaped surface, wherein a mixture containing a mixture of binders, a solids conveyor and a filler is sprayed onto the shaped surface by first producing a free jet for spray application from a mixture containing a mixture of binders and the solids conveyor, followed by metering the filler into the free jet to the incompletely polymerized mixture of binders. The method is suitable, in particular, for the spray application of heavy layers as employed in conventional mass-spring systems.

DE 10 2005 058 292 A1 describes a method and device for producing coated formed parts. Provided are a method and a device for producing formed parts containing a layer of polyurethane in shot operation, in which the reactive components are mixed by means of a cylindrical mixing chamber, and the reaction mixture produced subsequently flows through a flow channel and is sprayed onto the surfaces of a substrate, where it cures, followed by cleaning the flow channel by a gas stream.

Therefore, it is the object of the present invention to provide a process by which very different types of thermoplastically processable materials can be applied to different kinds of substrate surfaces in the form of a film in a desirable and defined way.

SUMMARY OF THE INVENTION

According to the invention, the above object is achieved by a method for spray-coating substrate surfaces, in which

-   -   b(a) a thermoplastically processable material is molten and thus         liquefied in an extruder in a first step;     -   (b) the molten material is pressurized by means of a carrier gas         or vapor;     -   (c) a mixture formed from said molten material and said carrier         gas or vapor is pressed through one or more nozzles, optionally         supplying a spraying gas having a temperature at least as high         as the melt temperature to the spraying jet in the region of the         discharge opening of the nozzle; and     -   (d) the obtained spray jet with the molten material is directed         onto the substrate surface, wherein the material impinges in         drops in a flowable state onto the substrate surface, forms a         continuous coating on the substrate surface, and subsequently         solidifies.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the invention, a method for spray-coating substrate surfaces, comprises the following steps :

-   -   (a) melting and thus liquefying a thermoplastically processable         material in an extruder in a first step;     -   (b) pressurizing by means of a carrier gas or vapor the molten         material;     -   (c) spraying a mixture formed from said molten material and said         carrier gas or vapor by pressurizing said mixture through one or         more nozzles, optionally supplying a spraying gas having a         temperature at least as high as the melt temperature to the         spraying jet in the region of the discharge opening of the         nozzle; and     -   (d) directing the obtained spray of mixture onto the substrate         surface, wherein the material impinges in drops in a flowable         state onto the substrate surface, thereby forming a continuous         coating on the substrate surface, which subsequently solidifies.

The core of the invention is essentially the fact that a thermoplastically processable material is molten alone or as a compound in a mixture with other thermoplastically processable materials with or without fillers in an extruder, pressurized with an inorganic carrier gas or vapor under a defined pressure, pressed as a mixture through one or more orifice nozzles, released to atmospheric pressure, and cooled down only on the substrate surface.

In a specific embodiment, a compound is directly mixed in a twin-screw extruder. In another embodiment, this twin-screw extruder is followed by, for example, a single-screw extruder or a melt pump.

Virtually all thermoplastically processable materials can be employed as materials for spraying, irrespective of whether they are homopolymers or compounds, unfilled or filled with non-melting materials. Particularly preferred within the meaning of the present invention are thermoplastically processable materials such as homopolymers, co- and terpolymers as well as thermoplastically processable elastomers, especially selected from acrylonitrile-butadiene-styrene (ABS), polyamide (PA), polylactate (PLA), polymethyl methacrylate (PMMA), polycarbonate (PC), polyethylene terephthalate (PET), polyethylene (PE), polypropylene (PP), polystyrene (PS), polyether ether ketone (PEEK) and polyvinyl chloride (PVC) including copolymers and compounds thereof, which may optionally contain further components, especially fillers. In a specific embodiment, high-performance materials, such as PEEK, may also be sprayed with a high proportion of gas and an associated viscosity reduction. The amount of the fillers can be varied widely, wherein the amount of the fillers preferably should not exceed 80% by weight, based on the material, because otherwise the cohesion of the coating cannot be ensured.

The fillers themselves may be inorganic in nature, or polymers that do not melt at the processing temperature, such as rubber, wherein the fillers do not have a preferred direction. Further, inorganic short fibers, polymeric fibers having a melting temperature higher than the processing temperature of the material or the compound as well as natural fibers may be employed as fillers.

A material structure that differs from layer to layer can be produced by changing the mixture, the thermoplastically processable material and/or the filler itself and the ratio of thermoplastically processable material to filler in the course of the spraying process.

Inert, especially inorganic, gases or mixture of gases as well as vapors suggest themselves as the carrier gas. Particularly preferred according to the invention is the use of, for example, nitrogen, carbon dioxide, air or water, which is gaseous at this temperature.

The carrier gas or vapor is preferably employed at a weight ratio of thermoplastically processable material to carrier gas or vapor within a range of from 100:0.1 weight parts to 100:30 weight parts, especially from 100:0.3 to 100:15 weight parts. Preferably, a spraying jet with a pressure of from 10 to 500 bar, especially from 20 to 400 bar, is sprayed.

With increasing pressure, the droplet size is reduced; the application of material becomes more homogeneous thereby. However, with increasing pressure, the material cools down more quickly upon expanding and relaxing. Depending on the material, this has the result that no bonding with the substrate surface is achieved any more. In this case, a pressurized heated gas (spray gas), preferably air, must be supplied, which provides for additional atomizing in a favorable case.

The nozzles are provided, for example, on a flexible pressure tube and can be moved over the substrate surface by means of robots to enable a complete application of material. The nozzle(s) itself (themselves) is (are) designed, for example, as (an) orifice nozzle(s) with 1 to 50 orifices, preferably 5 to 20 orifices, with a diameter of from 0.1 mm to 10 mm, preferably from 0.5 mm to 2 mm, or orifices having an equivalent cross-sectional area of defined orifice geometry of the mentioned orifice diameters. Hot air (spray gas) can be introduced for additional heating.

Slotted nozzles having dimensions of 0.1-3.0×3-30 mm, preferably 0.5-2.0×5-10 mm, may also be employed; in this case too, several slotted nozzles may be provided in the spray head.

In addition to the above mentioned substances and elements that are gaseous at room temperature (normal pressure) or at the processing temperature, the term “carrier gas” within the meaning of the present invention also includes those substances that form gaseous substances by a chemical or thermal reaction, or are converted to the gaseous state. Hot air is particularly preferred in this respect.

The chemical composition of the spray gas, which is preferably guided around the discharge opening of the thermoplast in the form of an annular nozzle, can be the same as or different from the carrier gas. Hot air is particularly preferred as a spray gas, which causes further expansion of the carrier gas in the thermoplast and heating of the particle surface.

In the following, the invention is illustrated by means of two Examples.

EXAMPLES Example 1

Commercially available KraussMaffei Berstorff extruders ZE40 Ax29D and KE90x3OD in series were employed, each equipped with specifically designed screws. The die was a single casing head with 24 orifice nozzles of 1.1 mm orifice diameter each, arranged in two rows.

A compound of 75% by weight inorganic filler (barite) and 25% by weight of a commercially available plastic mixture of PE/EVA, white oil, flow additive and temperature stabilizer in the form of granules was employed as the thermoplastically processable material.

The output capacity was 90 kg/h, and the gas quantity (carrier gas) was 1.1 kg/h CO₂. A commercially available pressed mixed fiber nonwoven served as the substrate surface.

With these settings, a good spray performance was achieved, and a flexible plastic layer was produced on the textile surface.

Example 2

In principle, essentially the same setting as in Example 1 was used in a second application in this Example. The nozzle has been changed. Here, a circular jet nozzle of the BETE company was used, in which heated air as a spray gas in an outer ring around the discharge nozzle heated the mixture of carrier gas and compound. The amount of carrier gas was reduced to 400 g/h. In the material employed, the filler was reduced to 25% by weight, and the amount of compound was increased accordingly.

The output capacity was 60 kg/h. The amount of air employed (spray gas) was 30 standard cubic meters per hour, heated at 300° C. The spraying result was improved over that of Example 1. 

1. A method for spray-coating substrate surfaces, in which (a) a thermoplastically processable material is molten and thus liquefied in an extruder in a first step; (b) the molten material is pressurized by means of a carrier gas; (c) a mixture formed from said molten material and said carrier gas is pressed through one or more nozzles, supplying a spraying gas having a temperature at least as high as the melt temperature to the spraying jet in the region of the discharge opening of the nozzle; and (d) the obtained spray jet with the molten thermoplastically processable material is directed onto the substrate surface, wherein the material impinges in drops in a flowable state onto the substrate surface, forms a continuous coating on the substrate surface, and subsequently solidifies.
 2. The process according to claim 1, characterized in that said thermoplastically processable material is selected from homopolymers, co- and terpolymers or thermoplastically processable elastomers, especially acrylonitrile-butadiene-styrene (ABS), polyamide (PA), polylactate (PLA), polymethyl methacrylate (PMMA), polycarbonate (PC), polyethylene terephthalate (PET), polyethylene (PE), polypropylene (PP), polystyrene (PS), polyether ether ketone (PEEK) and polyvinyl chloride (PVC) including copolymers and compounds thereof, which may optionally contain further components, especially fillers.
 3. The process according to claim 1 characterized in that an amount of fillers of from 0 to 80% by weight is employed, based on the thermoplastically processable material.
 4. The process according to claim 3, characterized in that said filler is inorganic in nature.
 5. The process according to claim 3 characterized in that said filler is in the form of an inorganic short fiber.
 6. The process according to claim 3 characterized in that said filler has polymeric fibers having a melting temperature higher than the processing temperature of the compound.
 7. The process according to claim 3 characterized in that said filler comprises natural fibers.
 8. The process according to claim 2 characterized in that the mixture, the thermoplastically processable material and/or the filler itself as well as the weight ratio of thermoplastically processable material to filler is varied in the course of the spraying process.
 9. The process according to claim 1 characterized in that nitrogen, carbon dioxide or air is employed as the carrier gas and/or spray gas.
 10. The process according to claim 1 characterized in that a spraying jet with a weight ratio of thermoplastically processable material to carrier gas within a range of from 100:0.1 weight parts to 100:30 weight parts is employed.
 11. The process according to claim 1 characterized in that a spraying jet with a pressure of from 10 to 500 bar is employed.
 12. The process according to claim 1 characterized in that from 1 to 50 nozzles with different output cross-sectional areas are employed.
 13. The process according to claim 1 characterized in that said mixture is pressed through orifice and/or slotted nozzles.
 14. The process according to claim 1 characterized in that orifice nozzles with the same or different diameters within a range of from 0.1 mm to 10 mm, or discharge openings with comparable cross-sectional areas are employed.
 15. The process according to claim 1 characterized in that slotted nozzles with the same or different cross-sectional areas of 0.1 to 3 mm×3 to 30 mm are employed.
 16. The process according to claim 1 characterized in that a continuous coating that is porous or coherent in itself is prepared.
 17. The process according to claim 1 characterized in that a substrate selected from textiles or a mold cavity with or without a surface structure is employed. 